Mixing Artificial Seawater - Tech Talk 27

Artificial seawater is a complex mix of many purified salts formulated to have a composition similar to natural seawater when dissolved in fresh water. Seawater cannot be evaporated to a dry salt and then be reconstituted by adding water. It just doesn’t work, so artificial blends were developed. There are many formulas, each with its own characteristics. The quality of a brand of sea salt is dependent on the formula, the quality of the raw materials and the uniformity of the blending. The salinity is the sum of all of the dissolved ions. Natural seawater is generally considered to have a salinity of 35 parts per thousand (ppt) or grams of salt per kilogram of water.

Several mistakes are common when calculating how much sea salt to use. First, do not assume that ppt is the same as grams per liter (g/L). Because seawater is heavier than fresh water, this assumption causes an error of about 2.5% lower salinity than expected. A second more serious error is to assume that the salt mix is anhydrous (water free), since all brands of sea salt use some hydrated salts (for cost and solubility reasons). This causes an error of about 12% lower salinity than expected. A third source of error is adding the salt to the desired volume, as the volume increases about 1% after adding the salt. What this means is adding 35 grams of sea salt to a liter of water (1000 grams) will result in a 1.01 liter solution at 29.1 ppt salinity!

Gallon designations for commercial brands are not calculated at 35 ppt. There is no standard. The weight per fifty gallon package varies among brands from 13.7 lbs to 16 lbs, giving salinities between 27 and 32 ppt. For calculation purposes, it takes 41.8 grams sea salt mix to one liter fresh water for 35 ppt salinity. To calculate salt needed for lower salinities, multiply the 41.8 g/L by the ratio of the desired salinity, divided by 35 ppt.